Transcript Dating with Radioactivity

Dating with Radioactivity
Chapter 12, Section 3
Basic Atomic Structure
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Each atom has a nucleus containing protons and
neutrons and that nucleus is orbited by electrons
Electrons have a negative electrical charge and
protons have a positive charge
Neutrons have no charge
The atomic number of an element is the number
of protons in the nucleus, atoms of the same
element always have the same atomic number
An atom’s mass number is the number of protons
and neutrons in a nucleus
The number of neutrons can vary in a nucleus,
and these variants, or isotopes, have different
mass numbers
Radioactivity
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The forces that bind protons and neutrons
together in a nucleus are usually strong
When nuclei are unstable, they spontaneously
break apart, or decay, in a process called
radioactivity
Radioactivity – the spontaneous decay of
certain unstable atomic nuclei
An unstable radioactive isotope of an element is
called the parent
The isotopes that result form the decay of the
parent are called the daughter products
Radioactive decay continues until a stable or
non-radioactive isotope is formed
Common
Types of
Radioactive
Decay
Half-Life
Half-Life – the time for one half of the
atoms of a radioactive substance to decay
to its stable isotope
 If the half-life of a radioactive isotope is
known and the parent/daughter ratio can
be measured, the age of the sample can
be calculated
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Radioactive Decay Curve
Concept Check
What is a half-life?
 The amount of time necessary for one half
of the nuclei in a sample to decay to its
stable isotope.
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Radiometric Dating
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Radiometric Dating – the procedure of calculating
the absolute ages of rocks and minerals that contain
radioactive isotopes
Each radioactive isotope has been decaying at a
constant rate since the formation of the rocks in which
it occurs
The products of decay have also been accumulating
at a constant rate
As uranium decays, atoms of the daughter product are
formed, and measurable amounts of lead eventually
accumulate
An accurate radiometric date can be obtained only if
the mineral remained in a closed system during the
period since its formation
Although the basic principle of radiometric dating is
simple, the actual procedure is quite complex
Radioactive Isotopes Frequently Used in
Radiometric Dating
Radioactive
Parent
Stable
Daughter
Product
Currently
Accepted HalfLife Values
Uranium-238
Lead-206
4.5 Billion
Years
Uranium-235
Lead-207
713 Million
Years
Uranium-232
Lead-208
14.1 Billion
Years
Rubidium-87
Strontium-87
47.0 Billion
Years
Potassium-40
Argon-40
1.3 Billion
Years
Concept Check
Why is a closed system necessary in
radiometric dating?
 An accurate radiometric date can be
obtained only if the mineral remained in a
closed system since its formation.
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Dating with Carbon-14
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Radiocarbon (Carbon-14) Dating – method for determining
age by comparing the amount of carbon-14 to the amount of
carbon-12
Carbon-14 is continuously produced in the upper atmosphere
It becomes incorporated with carbon dioxide, which is absorbed
by living matter
All organisms—including you—contain a small amount of
carbon-14; when an organism dies, the amount of carbon-14
gradually decays
By comparing the ratio of carbon-14 to carbon-12 in a sample,
radiocarbon dates can be determined
Because the half-life of carbon-14 is only 5730 years, it can be
used to date recent geologic events up to about 75,000 years
ago
Carbon-14 is a valuable tool to anthropologists, archeologists,
and historians
Production and Decay of Carbon-14
Concept Check
What is compared when dating with
carbon-14?
 The ration of carbon-14 to carbon-12.
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Importance of Radiometric Dating
Radiometric dating has produced dates for
thousands of geologic events in Earth’s
history
 Rocks formed on Earth have been dated
to be as much as 4 billion years old
 Meteorites have been dated at 4.6 billion
years old
 Radiometric dating has supported the
ideas of James Hutton, Charles Darwin,
and many others who inferred the geologic
time must be immense
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Assignment
Read Chapter 12, Section 3 (pg. 347-350)
 Do Section 12.3 Assessment #1-6 (pg. 350)
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